Microorganisms are exploited by man for the production of a wide range of products. Alcohols, antibiotics, vitamins, food supplements and many specialty chemicals are a few of the products derived principally from microbial production. In addition, microorganisms are crucial to domestic and industrial wastewater treatment and important within ecological and geochemical cycles. Bacteria have been used by the minerals industry to leach metals from ores by oxidative dissolution (Murr et al., 1976). In the copper mining industry, dump leaching has been carried out on a large scale using the rod-shaped bacterium Thiobacillus ferrooxidans (Brierley, 1982). The exploitation of microorganisms to perform specific and otherwise difficult industrial tasks will most likely increase as progress in genetic manipulation continues.
Among the many remarkable and potentially very useful microbial transformations are those that are geologically important. The study of microbial biogeochemistry has defined many bioredox, bioprecipitation, and biosolution reactions and has resulted in classification of microorganisms responsible for these processes.
A number of microorganisms catalyze the dissimilative reduction of iron ores (Ehrlich, 1981). Commercial application of microbial extraction, from a systematic exploitation of these organisms and biologically catalyzed reactions would be of great benefit to the metal-producing industry. Commercial extraction of iron ore by reduction of ferric to ferrous iron through microbial action would be a boon to the steel industry.
The number of bacteria whose iron-reducing capabilities are established is fairly large. However, there are significant differences among species and classes of species in such bacteria, in terms of both efficiency and kinetics of iron reduction and the environmental conditions under which the transformation is observed.
The cost of employing microbial methods for solubilizing iron(II) from ore must be comparable to that of existing industrial methods of iron extraction and reduction if the process is to be commercially viable. In 1978, the cost of producing carbon steel in the U.S. and Japan was approximately $400 per ton. It is interesting to note that the most rapidly escalating component cost was that of materials. While still higher than in other countries, the cost of labor for steel production in the U.S. rose more slowly between 1969 and 1978 than in other major western steel producing nations. The reported average U.S. cost for iron ore was about $44.51 per ton of steel produced. It is assumed that this figure represents the total cost of extracting and pelletizing iron oxides in a form suitable for production of pig iron. Considering the general inflation rate observed during the intervening years, a similar cost today could easily be twice as high.
In 1976, the unit cost of coking coal in the U.S. was $53.73 per ton of steel produced. During steel production, coke is introduced to maintain reducing conditions necessary for production of elemental iron from iron oxides. Prereduction of iron(III) oxides to iron(II) would result in significant savings in coke demand. Our preliminary economic analysis has lead us to believe that microbial extraction and beneficiation of iron ore can be economically attractive. It is an object of this invention to utilize microorganisms for the reductive dissolution of iron from iron ore. It is a further object to carry out such reduction at a cost comparable to conventional coke/steel processes.
Dissimilative iron reduction has been explored by others. FIG. 1 is a graphic summary of what we believe are the most relevant published kinetic data. Iron reduction rates observed by Obuekwe and Westlake in cultures of Pseudomonas sp. 200, FIG. 1A are by far the most rapid, while those measured by Brock and Gustafson in cultures of Sulfolubus acidocaldarius, FIG. 1B and by Kino and Usami in cultures of Thiobacillus thiooxidans, FIG. 1C, are appreciably slower. The data of FIG. 1A represent iron solubilization by Pseudomonas sp. 200 grown anaerobically on complex medium at 30.degree. C.
The data of FIG. 1B represent aerobic reduction of ferric iron by S. acidocaldarius at 70.degree. C. using elemental sulfur in one experimental set, and glutamate in another set as the energy source. The data of FIG. 1C represent aerobic reduction of ferric iron by T. thiooxidans using elemental sulfur as the energy source at room temperature in one experimental set and at 30.degree. C. in another set. Under experimental conditions employed, the maximum observed rates of iron reduction were 50 and 90 mg per liter-day, respectively.
The following references, some of which are referred to herein, may be of further interest to the reader.
Atkinson, Bernard and Ferda Mavituma (1983) Biochemical Engineering and Biotechnology Handbook, Macmillan Publishers, Ltd. PA0 Brierley, C. L. (1982) Microbiological mining, Scientific Am. 247, 44-53. PA0 Brock, T. D. and J. Gustafson (1976) Ferric iron reduction by sulfur and iron-oxidizing bacteria, Appl. Environ. Microbiol. 32, 567-571. PA0 Ehrlich, H. L. (1981) Geomicrobiology, Marcel Dekker, New York, p. 187. PA0 Kino, K. and S. Usami (1982) Biological reduction of ferric iron and sulfur oxidizing bacteria, Agric. Biol. Chem. 46, 803-805. PA0 Metcalf & Eddy, Inc. (1979) Wastewater Engineering: Treatment, Disposal, Reuse, 2nd edition, McGraw-Hill, New York. PA0 Murr, L. E., A. E. Torma and J. A. Brierley (1978) Metallurgical Applications of Bacterial Leaching and Related Phenomena, Academic Press, New York, 526 pages. PA0 Obuekwe, C. O. and W. S. Westlake (1982) Effects of medium composition on cell pigmentation, cytochrome content, and ferric iron reduction in a Pseudomonas sp. isolated from crude oil, Can. J. Microbiol. 28, 989-992. PA0 Stryer, Lubert (1981) Biochemistry, 2nd edition, W. H. Freeman and Company, San Francisco.